Postcombustion system and method for operating a postcombustion system

ABSTRACT

An afterburner device and a method for operating an afterburner device, especially for chemical reformers for obtaining hydrogen, for making heat available from fuels and/or residual gases from a reforming process and/or a fuel cell process. In this context, heat is supplied in a controlled manner from recirculated combustion gases to a first housing and/or a combustion chamber situated in it and at least in part filled with heat resistant, open-pored foamed ceramics. The regulation takes place, for instance, based on a temperature recorded in the combustion chamber using an infrared light measurement.

FIELD OF THE INVENTION

The present invention relates to an afterburner device, and to a methodfor operating an afterburner device.

BACKGROUND INFORMATION

In fuel cell-supported transportation systems so-called chemicalreformers are used for obtaining the required hydrogen fromhydrocarbon-containing fuels.

The optimal operating temperature of a chemical reformer is usually farabove its surrounding temperature. Particularly in vehicles for personaltransportation, this leads to problems. The numerous standstill phasesof the vehicle lead to a large number of cold start phases, in whichespecially the chemical reformer does not function optimally. At veryslight load, the reformer also does not, under certain circumstances,achieve the optimal operating temperature by the heat arising in it, orloses it during the operation.

Therefore, particularly in the case of fuel cell supported drive systemshaving a chemical reformer, it may be of advantage to installafterburner devices which rapidly bring the chemical reformer tooperating temperature using the heat produced by it, and/or of usingaccumulated residual gases thermally.

An afterburner device burns the combustible residual gases, such asresidual hydrogen, while forming flames and/or at least partiallycatalytically, and is thermally coupled to the chemical reformer.However, the heat energy of the combustible residual gases is generallynot, by itself, sufficient for making available a sufficiently greatheat output. That is why generally, in addition or by itself, fuel ismetered into the afterburner device. In this context, the fuel, which ispreferably present in liquid form, is injected into a combustionchamber, finely divided, by devices that are costly and subject toerror, as a cloud of droplets having a droplet diameter that is as smallas possible. The slight droplet diameter is required in order to bringthe fuel into contact with oxygen and heat over as large a surface aspossible, and in order thus to carry out the combustion process ascompletely as possible.

In this context, it may be a disadvantage that metering devices forgenerating a cloud of droplets having a small droplet diameter are verycostly, cost intensive and subject to error. The required small dropletdiameter is often able to be achieved only by using high fuel pressure,the generation of high pressure requiring a relatively high quantity ofpower, and in particular, the equipment for generating the pressurerequiring much space. In addition, such metering devices usually havevery small metering apertures, which, by combustion residues ordeposits, change the metering behavior of the metering device in animpermissible and poorly controllable way. Alternatively to, orsupportive of the application of high fuel pressure, solutions havingair support are known for the fine atomization of the fuel, the fuel orthe residual gas being intermingled sufficiently long with air beforethe combustion. In this connection, the disadvantages are the largespace requirement, the control of the air metering that is costly andsubject to interference, and the additional energy requirement.

Finally, especially at low power, the danger arises of unexpectedextinguishing of the open continually burning flame in the combustionchamber. The heat output of the afterburner device is therefore stronglyrestricted in the downward direction. Furthermore, there is alwaysrequired a certain amount of time for shutting off the fuel supply orreigniting the flame. During this time, the fuel and the residual gasare able to collect in the combustion chamber. This negativelyinfluences the reignition, a catalytic converter that may possibly bepresent may be damaged, and uncombusted fuel and residual gas may escapeinto the atmosphere. In spite of all the measures named, uncombusted orincompletely combusted portions remain behind in the exhaust gas of theafterburner device, which are, in part, poisonous or chemicallyaggressive. This leads to increased environmental loading and materialloading, and besides all that, the caloric value of the fuel or theresidual gas is utilized only incompletely.

SUMMARY

A method according to an example embodiment of the present invention andan afterburner device according to an example embodiment of the presentinvention, may have the advantage that, because of the metering of fuelonto or into the open-pore, heat resistant foamed ceramics without theapplication of costly atomization devices for generating the finest fueldrops, a very good fuel distribution takes place in the combustionchamber and in the foamed ceramics. The relatively high contact surfacewith air oxygen that goes along with this, leads to an almost completecombustion of the fuel and residual gas supplied, and thus to anexcellent efficiency and very low pollutant emissions. The demands onthe metering device and the fuel nozzle that meter the fuel into thecombustion chamber or into the foamed ceramics are very low, since thedistribution of the fuel takes place within the foamed ceramics.

Because of the low heat capacity of the foamed ceramics and because ofthe combustion process that is distributed uniformly and spaciously inthe foamed ceramics, the foamed ceramics heat up very rapidly, as aresult of which, even after a brief operating duration and possiblyoccurring brief interruptions of the fuel supply, an externally suppliedignition, such as by spark plugs, is not necessary upon resumption ofthe fuel supply. The utilization of the exhaust gas heat by therecirculation of the exhaust gases created in the combustion via arecirculating line and a heat exchange channel, which heats with exhaustgas heat the supplied air and/or the combustion chamber and the foamedceramics, especially during a cold start operation, leads to anabbreviated cold start phase and consequently to an additional reductionof pollutant emissions as well as to an additional improvement in fuelconversion. By recording the combustion speed, it is possible toregulate the recirculated quantity of heat. Thereby it is possible, inthe cold start phase, to recirculate the largest measure of heatquantity without generating unfavorable temperatures for the afterburnerdevice or its operation, in response to increasing combustion speed.

It may also be advantageous that the foamed ceramics first accommodatesa part of the metered fuel without its being immediately ignited.Rather, a part of the fuel is first distributed in the foamed ceramics,before it is ignited at its surface. Thus, the foamed ceramics are in aposition of first storing a certain quantity of fuel. Thischaracteristic, for example, is of advantage in response to the startupof the afterburner device from a cold state and in response to onlyinsufficient external ignition by, for example, a coiled filament, sincethe fuel is not immediately able to escape uncombusted all the waythrough the combustion chamber.

Rather, it is stored in the foamed ceramics, and is further availablefor combustion. Deflagration in the combustion chamber or an enrichmentof the fuel/air mixture beyond ignitability are consequently largelyavoided.

Furthermore, it may also be of great advantage that the distribution ofthe fuel primarily takes place automatically, largely independently ofthe geometrical shaping of the foamed ceramics. This permits a veryadaptable placing of the foamed ceramics in the combustion chamber andthe afterburner device, so that one may improve, for example, thethermal coupling between foamed ceramics and combustion chamber, or withother elements of the afterburner device.

In addition, the afterburner device according to the present inventionhas a very large heat output range, which comes about particularly fromthe possibility of setting very small heat outputs. Because of thesevery small heat outputs or fuel powers that may be set, it is possibleto avoid switching on and off procedures of the afterburner device thatare pollutant intensive, material stressing and efficiency lessening,especially in response to load changing procedures that are typical ofpersonal automobile transportation.

In one first advantageous improvement of the method according to anexample embodiment of the present invention, the combustion speed isestablished in the light of a temperature measurement. Especiallyadvantageously, this may be done using a contactless and thus largelywear-free infrared light measurement.

In one additional advantageous development, the quantity of recirculatedcombustion gases is regulated based on the determined combustion speed.

The method according to the present invention may also advantageouslyimproved by an additional method step which regulates the supply of air,fuel and/or residual gas as a function of the recorded combustion speed.In an additional further refinement the supply of air into thecombustion chamber or the air proportion of the fuel/gas-air mixturebeing increased in order to reduce the temperature in the combustionchamber or in the foamed ceramics.

The method may also advantageously refined if the method also has amethod step in which the combustion chamber or the foamed ceramics areelectrically heated. Thereby the combustion chamber or the foamedceramics may, for example, be heated even before the beginning of thecold start phase, whereby the cold start phase of the afterburner deviceis abbreviated still further. Similarly, in this manner, the respectiverequired ignition energy may be made available, or a required ignitionenergy may be generated.

The afterburner device may be advantageously refined in that the foamedceramics are made at least partially of silicon carbide. Silicon carbideis outstandingly heat resistant, an excellent heat conductor and, beyondthat, it provides the foamed ceramics with a good mechanical rigidity ata relatively slight thickness. Besides, silicon carbide is a relativelygood conductor of electric current. The good electrical conductivity maybe utilized for measuring technology purposes, in order, for instance,to determine the temperature via the electrical resistance derived fromthe current and the voltage, or the combustion procedure may especiallybe influenced or controlled via the heat effect of the electric currentor, in the case of partial load operation, be completely achieved, e.g.,during catalytic combustion.

It may also be advantageous if the foamed ceramics are made in anopen-pored manner, by so-called reticulation, which may be done, forinstance, in a thermal or chemical manner. Thereby a high degree of openporosity may be achieved, and in addition, the pore size may very easilybe set in the range of 0.05 mm to 5 mm, during the manufacture of thefoamed ceramics.

Preferably, the foamed ceramics are in good heat conductive contact withat least one part of the wall of the combustion chamber or the firsthousing, since thereby the heat may be passed on rapidly and efficientlyto, for instance, the reformer or to a fuel cell. Similarly, thecombustion chamber or the foamed ceramics may be heated from the outsideby this wall of the combustion chamber or of the first housing, forexample, by a recirculated exhaust gas stream, without having theexhaust gas reach into the combustion chamber or the foamed ceramics.

Because of the positioning of heat conducting elements within the firsthousing, especially also within the foamed ceramics, it isadvantageously possible to conduct heat from a relatively hot regioninto a region that is relatively cool compared to it, especially intothe region of air supply or into the region in which the fuel or theresidual gas are metered in. Thereby, the cooling effect of the suppliedreactants is compensated for and the speed of the reaction, especiallyin the cold start phase, is increased in the regions mentioned. Thespeeds of the reactions therefore run uniformly in all regions of thecombustion chamber or the foamed ceramics. It is of special advantage ifthe heat conducting elements are made of metal or a metal-containingalloy, since metals are especially good heat conductors, and also havegood mechanical and chemical properties.

Furthermore, it is advantageous to recirculate the heat energy of thehot exhaust gases by a recirculating line and a heat exchange channel tothe foamed ceramics or the combustion chamber and/or the supplied air,and thereby to the combustion reaction itself. Thereby the qualitativelylow-value exhaust gas heat is utilized, especially in cold start phases,in order quickly to increase the speed of combustion, and in order topreheat the reactants or the combustion chamber.

It is also of advantage if a controller regulates or controls therecirculation of the exhaust gases. It is therefore advantageouslypossible to recirculate hot exhaust gases in a metered way, and thus toadjust the recirculated quantity of heat to the heat demand. Inparticular, thereby, overheating of the afterburner device is avoided,and the back pressure of the exhaust gas is held as low as possible.

Furthermore, it is advantageous to manufacture the heat exchangechannels from cylindrical tubes, since these are cost-effective and easyto process.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention are explained in greaterdetail in the following description and are shown simplified in thefigures.

FIG. 1 shows a schematic representation of a first exemplary embodimentof an afterburner device according to the present invention, as adiagrammatic sketch.

FIG. 2 shows an excerpted schematic representation of a second exemplaryembodiment according to the present invention in the region of thecombustion chamber.

FIG. 3 shows an excerpted section through the open-pored foamedceramics, as a diagramatic sketch.

FIG. 4 shows a schematic representation of a third exemplary embodimentaccording to the present invention.

FIG. 5 shows a schematic sectional representation of the third exemplaryembodiment, according to the present invention, in a top view.

FIG. 6 shows a schematic representation of a fourth exemplary embodimentaccording to the present invention.

FIG. 7 shows a schematic representation of a fifth exemplary embodimentaccording to the present invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In the following, exemplary embodiments of the present invention aredescribed by way of example. The example method according to the presentinvention is particularly advantageously used on these exemplaryembodiments. Identical parts are provided with the same referencenumerals in all the figures. The arrows represent the respective fueland gas flows.

FIG. 1 shows an exemplary embodiment of an afterburner device 1according to the present invention. The afterburner device has acylindrical tubular second housing 14 closed at the ends, a nozzle 2, acontroller 17 and a recirculating line 16. Nozzle 2 penetrates into thetop end face of second housing 14, and is positioned axially centricallyto an axis 22, which in this exemplary embodiment is identical to theaxis of symmetry of second housing 14. The top end face of secondhousing 14 also has an air supply 3 which, in this exemplary embodiment,is implemented as just an opening. At the side, close to the lower endof second housing 14, there is a discharge opening 7, which opens outinto an outlet pipe 15. Outlet pipe 15, a first exhaust gas line 20 andrecirculation line 16 open out into controller 17. Recirculation line 16leads from controller 17 to the upper end face of second housing 14, andthere opens out into second housing 14. In the interior of secondhousing 14 there is, among other things, a combustion chamber 8, whichis not shown in FIG. 1.

The example method of functioning is as follows:

Through nozzle 2, either only fuel in preferably liquid form, onlyresidual gas from, for instance, a reforming process or a fuel cellprocess, or a mixture of these two cobustible substances is metered intocombustion chamber 8, that lies in second housing 14 and is not shown inFIG. 1. The air required for the combustion is aspirated through airsupply 3. However, a forced feed of air or other oxygen-containingsubstances is possible.

The exhaust gases exiting at discharge opening 7 flow through outletpipe 15 into controller 17, and are recirculated at least in part viarecirculation line 16 into second housing 14. The recirculated exhaustgases give off heat energy to the interior of second housing 14, withoutthe recirculated exhaust gases mixing with the fuel, the residual gasesor the air, and are conveyed via a first exhaust gas line 19, not shownin FIG. 1, into the environment or into another process. The exhaustgases not recirculated by controller 17 are conducted by controller 17through a second exhaust gas line 20 into the environment or intoanother process.

In this exemplary embodiment, the instantaneous temperature or theinstantaneous temperature distribution in the interior of second housing14 or combustion chamber 8, that is not shown, is measured by infraredsensors that are not shown. Thereby one may, in particular, establishthe instantaneous speed of combustion in combustion chamber 8. In thisexemplary embodiment, as a function of the speed of combustion, thequantity of the exhaust gases recirculated into second housing 14 isregulated. The respective proportions and quantities of air, fuel andresidual gases, which reach second housing 14 through nozzle 2 orthrough air supply 3 are also regulated according to the presentinvention as a function of the speed of combustion. It is also possiblethat one might change the respective proportions and quantities of air,fuel and residual gases in a time-controlled manner. Thus, for example,at the beginning of a cold start, overall fewer reactants are suppliedand the fuel proportion is increased, at a later point in time, forexample, the air proportion being raised, and the quantity of reactantsbeing raised overall. At too high a temperature, according to thepresent invention, the air supply or the air proportion in combustionchamber 8 is increased.

In addition, a lambda sensor may also be present.

FIG. 2 shows a schematic representation in excerpted form of a secondexemplary embodiment according to the present invention in the region ofcombustion chamber 8, which is situated in second housing 14 that is notshown in this figure. Combustion chamber 8 is bordered at the side by acylindrical tubular first housing 5, at the top by an upper ring 9 andat the bottom by a lower ring 10 in housing 5. Upper ring 9 separatescombustion chamber 8 from a nozzle 2, and lower ring 10 separatescombustion chamber 8 from an outlet chamber 11. Combustion chamber 8 is,in this exemplary embodiment, filled completely with foamed ceramics 4.The pores of the foamed ceramics are connected to one another in thetransverse direction and the longitudinal direction, and thusparticularly permit an outstanding flow-through and nearly completecombustion. The surface of foamed ceramics 4 is, in this exemplaryembodiment, completely coated with a catalytic layer made of CuO.

An excerpted section is shown in FIG. 3 as a diagrammatic sketch. Pores13 that are embedded in carrier foam 12 may be recognized there.

The foamed ceramics may be made, for example, by reticulating carrierfoam 12, such as polyurethane foam, and subsequent treatment with asilicon carbide suspension, for instance, a ceramic powder made ofsilicon carbide suspended in water. Nozzle 2 takes up fuel, residualgas, air or a mixture of these components at its axial end facing awayfrom foamed ceramic 4, and meters them in, at its lower axial end facingfoamed ceramics 4, through an opening, that is not shown, into foamedceramics 4. Air is also supplied via an air supply 3 to combustionchamber 8 or to the combustion. Introducing a residual gas/air mixtureor a residual gas/oxygen mixture is also possible via air supply 3.Fuel, residual gas or a mixture of these components ignites with airand/or oxygen or reacts chemically in running operation at the hotsurface of foamed ceramics 4.

However, the combustion process may also be started or kept going byignition devices that are not shown. Such ignition devices are, forexample, applied as an electrical glow plug or coiled filament betweennozzle 2 and foamed ceramics 4. It is also possible to apply theignition device in foamed ceramics 4. It is also possible to design theignition device in such a way that the whole foamed ceramics 4, or atleast a part of it, is electrically heated in such a way that this formsan ignition device. Finally, foamed ceramics 4 may also be heated fromthe outside or by installing wires. This makes possible the operation ofafterburner device 1 according to the present invention.

After the oxidation of the fuel and/or the residual gas has taken place,the combustion gases escape downwards through lower ring 10 into outletchamber 11, in order then to escape via discharge openings 7.

First housing 5 is in good heat-conductive contact over a large areawith heat exchange channels 18, that are not shown in this figure.

In the interior of foamed ceramics 4, there run strip-shaped heatconducting elements 23. They may also be shaped, for instance, as tubesor cylindrical tubes. In this exemplary embodiment, heat conductingelements 23 run from top to bottom, parallel to axis 22. They are usedto transport heat into regions inside first housing 5 which, forexample, in a cold start phase, heat up only slowly relative to otherregions within first housing 5. In such a way, for example, heat may beconducted from one region near lower ring 10 into a region near upperring 9. At least a part of heat conducting elements 23 may alsopenetrate through upper ring 9, and thus, for example, heat the airsupplied by air supply 3, and similarly they may penetrate through lowerring 10, in order to conduct away heat energy from the combustion gases.Heat conducting elements 23 should be positioned in such a way that theyare not directly acted upon by the fuels metered in through nozzle 2.

FIG. 4 shows a third exemplary embodiment according to the presentinvention that is similar to the exemplary embodiment in FIG. 2.However, this exemplary embodiment additionally has recirculating line16, which conducts the combustion gases via discharge opening 7, outletpipe 15 and controller 17 into the lower end of heat exchange channels18. As may be seen in FIG. 5, heat exchange channels 18, in theexemplary embodiment, of FIGS. 4 and 5 run in one-half of a tube 21formed as a hollow cylinder that is closed at one end. Tubes 21 run frombottom to top along the lateral wall of first housing 5 and arethermally coupled with combustion chamber 8 and first housing 5. Tube 21is divided into two halves by a tube wall 24 that divides the tube crosssection, the half facing first housing 5 representing heat exchangechannel 18, and the half facing away from it representing a firstexhaust gas line 19. Tube wall 24 runs to shortly before the closed endof tube 24 in order to create a connection between heat exchange channel18 and first exhaust gas line 19. Other than that, it hermeticallyseparates the two halves of tube 21. Tubes 21 are distributed radiallyabout first housing 5, at uniform distances. Tubes 21 and first housing5 are surrounded by second housing 14, the lateral walls of secondhousing 14 act especially in a heat insulating manner.

Controller 17 determines the quantity of recirculated combustion gases,and conducts them via recirculating line 16 to the lower end of tube 21into heat exchange channels 18. For example, in a cold start phase, theheat contained in the combustion gases is supplied to first housing 5and thus to combustion chamber 8 and the chamber lying above the upperring. The speed of combustion may therefore be accelerated in a coldstart phase and thereby the cold start phase may be shortened. Inparticular, because of the heat supplied, the metered in fuel is able tovaporize more easily and more rapidly. The combustion gases then leaveafterburner device 1 via first exhaust gas line 19. The combustion gasesthat are not recirculated are also conveyed out of afterburner device 1via a second exhaust gas line 20.

FIG. 6 shows a fourth exemplary embodiment similar to the thirdexemplary embodiment shown in FIGS. 4 and 5. Recirculating line 16,however, subdivides the recirculated combustion gases into heat exchangechannels 18, which run through combustion chamber 8 and foamed ceramics4. Heat exchange channels 18 are shaped cylindrically tubular; and theyrun through the lateral walls of first housing 5. A second housing 14 isnot present.

FIG. 7 shows a fifth exemplary embodiment according to the presentinvention, along with first housing 5 situated in second housing 14. Therecirculated combustion gases are guided via recirculating line 16through a first opening 25 situated in second housing 14, and are guidedto a second opening 26 of second housing 14, through heat exchangechannel 18 that is formed between the two housings. There, thecombustion gases leave afterburner device 1 via first exhaust gas line19. First housing 5 is hermetically sealed from the recirculatedcombustion gases, and takes up heat from the recirculated combustiongases, for instance, in a cold start phase. Thereby, combustion chamber8 and foamed ceramics 4, which are situated in the first housing, areheated up.

1-20. (canceled)
 21. A method for operating an afterburner device forthe afterburner device having a nozzle for metering in at least one offuel, residual gases, and air, into a combustion chamber that is filledat least in part with foamed ceramics, and having a discharge openingfor discharging combustion gases, the method comprising: recording aspeed of combustion in at least one of the combustion chamber and thefoamed ceramics; recirculating at least a part of the combustion gasesto a heat exchange channel that is thermally coupled to at least one ofthe combustion chamber and the foamed ceramics; and regulating aproportion of the recirculated combustion gases by changing a quantityof the recirculated combustion gases.
 22. The method as recited in claim21, wherein the recording step includes measuring a temperature.
 23. Themethod as recited in claim 22, wherein the temperature is measured viaan infrared light measurement.
 24. The method as recited in claim 21,wherein the quantity of the recirculated combustion gases is regulatedbased on the speed of combustion in the at least one of the combustionchamber and the foamed ceramics.
 25. The method as recited in claim 21,further comprising: regulating a supply of the at least one of the fuel,residual gas, and air, as a function of the recorded speed ofcombustion.
 26. The method as recited in claim 25, wherein at too high atemperature or too great a speed of combustion, a supply of air isincreased.
 27. The method as recited in claim 21, further comprising:electrically heating at least one of the combustion chamber and thefoamed ceramics.
 28. An afterburner device for making heat availablefrom a fuel or residual gase from a reforming process or from a fuelcell process, the afterburner device comprising: a first housing; acombustion chamber situated in the first housing; at least one nozzlefor metering in fuel or residual gases into the combustion chamber; andat least one air supply; wherein the combustion chamber is filled atleast in part with heat resistant, open-pored foamed ceramics, which iscoated at least in part with a catalytic material.
 29. The afterburnerdevice as recited in claim 28, wherein the afterburner device is for achemical reformer for obtaining hydrogen.
 30. The afterburner device asrecited in claim 28, wherein the catalytic material is made up of atleast one of ZnCuO and CuO.
 31. The afterburner device as recited inclaim 28, wherein the foamed ceramics are made at least in part ofsilicon carbide.
 32. The afterburner device as recited in claim 28,wherein the foamed ceramics are made open-pored by reticulating.
 33. Theafterburner device as recited in claim 28, wherein the foamed ceramicsare electrically heatable.
 34. The afterburner device as recited inclaim 28, wherein the foamed ceramics are in good heat-conductivecontact with at least a part of the first housing.
 35. The afterburnerdevice as recited in claim 28, wherein heat conducting elements runwithin the first housing.
 36. The afterburner device as recited in claim35, wherein the heat conducting elements are made up of metal or a metalalloy, and run in the foamed ceramics or in the region of the airsupply.
 37. The afterburner device as recited in claim 28, furthercomprising: at least one recirculating line to recirculate combustiongases created during combustion into at least one heat exchange channel,the heat exchange being thermally coupled to the combustion chamber orthe foamed ceramics, and configured to guide exhaust gas heat into atleast one of the combustion chamber, the foamed ceramics, and a regionof the air supply.
 38. The afterburner device as recited in claim 37,further comprising: a controller that regulates or controls therecirculating of the combustion gases created during the combustion intothe at least one heat exchange channel.
 39. The afterburner device asrecited in claim 37, wherein the at least one heat exchange channel ismade up of tubes.
 40. The afterburner device as recited in claim 39,wherein the tubes have a hollow, cylindrical shape.
 41. The afterburnerdevice as recited in claim 37, wherein at least a part of the heatexchange channels is situated radially about the combustion chamber,parallel to an axis.
 42. The afterburner device as recited in claim 37,wherein at least a part of the heat exchange channels runs through thecombustion chamber or the foamed ceramics.
 43. The afterburner device asrecited in claim 37, wherein the at least one heat exchange channelguides the exhaust gas flow at least of onto and about the firsthousing.